This invention relates to an overedge sewing machine for cutting the edge of a fabric and sewing an overedge stitch at the same time.
The U.S. Pat. No. 4570558 discloses one known overedge sewing machine. In this prior-art overedge sewing machine, a lower knife holder supporting a lower knife crosses the fabric-feed direction at a right angle. When an adjustment knob is rotated, an external screw is engaged with an internal screw, the lower knife holder is moved, and stitch width is thus adjusted. A stitch-support member is fixed on the lower knife holder. When the stitch width is altered, the stitch-support member is moved together with the lower knife holder.
Generally, an excess amount of upper and lower looper thread is necessary for the thread to reciprocate between the needle location and the fabric edge. Excess thread in proportion to the stitch width is also necessary. In this prior art, an adequate amount of reciprocating thread corresponding to the stitch width can be obtained. However, since the stitch-support member moves together with the lower knife and the relative position of the stitch-support member with the lower knife does not change, an adequate amount of excess thread proportional to the stitch width cannot be obtained. The amount of thread is so insufficient that the stitch becomes too taut when the adjusted stitch width increases.
Another known overedge sewing machine is disclosed in the U.S. Pat. No. 4690079. In this prior-art overedge sewing machine, the lower knife holder supporting the lower knife crosses the cloth feed direction at a right angle. When the adjustment knob is rotated, a movable member is moved and the external screw is engaged with the internal screw. The lower knife holder is moved accordingly, and the stitch width is thus adjusted. The stitch-support member attached to the movable member crosses the fabric-feed direction at a right angle. After the stitch width is adjusted, the position of the stitch-support member can be finely adjusted by loosening a set screw.
In this prior art, when the stitch width is altered, the position of the lower knife holder should be adjusted, then that of the stitch-support member should be finely adjusted. On the other hand, the movable member of the lower knife holder supports the stitch-support member, so the stitch-support member moves together with the lower knife holder when the stitch width is altered. Every time the stitch width is adjusted, the position of the stitch-support member must be finely adjusted according to the type of the fabric.